Light leaf spot risk highest in central Britain

AHDB’s light leaf spot (LLS) preliminary forecast shows that the disease risk to winter oilseed rape is relatively high across central Britain.

The regional disease forecast highlights the proportion of oilseed rape crops with a disease resistance rating of 5 predicted to have more than 25% of plants affected by the spring.

Risks

For southern England, the risk is slightly higher than forecast during 2018-19 but remains relatively low.

Historically, the risk is more serious toward the north of Britain. However, the 2019-20 forecast shows that the risk is moderate for much of Scotland.

Jon West, who manages the forecast at Rothamsted Research, said: “A relatively warm summer and low disease levels at the end of last season have driven down the light leaf spot risk across Scotland. In fact, you have to go back to the 2013-14 season to find a lower-risk year.

Most of the Midlands, Wales and the north of England is forecast to be at a relatively high risk of this disease. However, this risk level could not be described as unusually high for these areas.

The forecast can guide crop management decisions. For a more accurate picture of local risk, people can build a customised forecast via the AHDB website.

This forecast also takes account of variety, sowing date and autumn fungicide applications.

For the strongest risk assessment, farmers should place leaf samples in polythene bags and incubate them at 10–15°C, for about five days, to bring out symptoms.

The preliminary forecast uses previous season pod incidence data and deviation from the 30-year mean summer (July/August) temperature data. In spring, an update to the forecast reflects deviation in winter rainfall from the 30-year mean.

To access information on LLS and fungicide performance visit: ahdb.org.uk/lightleafspot.

Symptoms

Symptoms are uncommon until late autumn or early winter. On green leaf tissue, very small white spots (spore masses) are visible. These develop into discrete lesions with pink-tinged centres.

In the most severe cases, whole leaves can be killed. During the winter, severely affected plants may also be killed. Surviving plants can become stunted or distorted.

When the disease spreads to stems and lateral branches, elongated, fawn lesions, surrounded by black speckling, can appear.

Under humid conditions, white spore masses can also form. Under conducive conditions, the disease can spread to and distort pods. These may turn brown and shatter prematurely.